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Key with
Final Cut Pro
There are two different methods used for
keying: chroma keying and luma keying. Chroma keying is a method of
keying on a particular hue of color. Although any color can be keyed on,
the colors most frequently used for chroma keying are blue and green.
Specific hues of blue and green with particular levels of saturation
have been developed that provide the best results, and different
companies have created commercially available paints, fabrics, and
papers that use these colors. The color you use—blue or green—depends
largely on the color of your foreground subject. If you’re trying to
create a key around a blue car, you probably want to use green as your
background. Another advantage of using green, when possible, is that
video formats generally preserve more information in the green component
of the signal, resulting in slightly better keys. Luma keying is based
on a particular range of luminance. Black is usually used, but you can
also key on white. While keying out a white or black background may be
more convenient in certain circumstances, it may be harder to correctly
isolate your foreground subject because of shadows and highlights, which
may have black or white values close to the luma range you’re
keying out.
Chroma Keyer
Allows you to create a key using any range of color you want, including
(but not limited to) the usual blue and green. You can also fine-tune
your composite by adjusting the color value, saturation, and luminance
ranges used to define your key, together or separately. For example, if
you only want to perform a luma key, you can disable color and
saturation. Even when performing a color key, you’ll get superior
results by manipulating the Color Range and Saturation controls
separately. Color Key Keys on any color in a clip. Color controls allow
you to select a color from your clip as the specified key color.
Sometimes referred to as chroma key.
Color
Smoothing - 4:1:1
Color Smoothing - 4:2:2
Improves the quality of chroma keys and reduces diagonal
“stair-stepping” that can occur in video clips with areas of
high-contrast color. Use 4:1:1 Color Smoothing with NTSC or PAL DV-25
video sources. (The exception is PAL mini-DV/DVCAM, which uses 4:2:0
color sampling.) Use 4:2:2 Color Smoothing for DVCPRO 50, DVCPRO HD, and
8- and 10-bit uncompressed video. To improve the quality of your chroma
key, apply the appropriate smoothing filter to the clip you want to
chroma key first. As you add additional keying filters, make sure that
the Color Smoothing filter remains the first one in the video section of
the Filters tab.
Luma Key
Similar to a chroma (color) key, except that a luma key creates a matte
based on the brightest or darkest areas of an image. Keying out a
luminance value works best when your clip has a large discrepancy in
exposure between the bright or dark areas in the frame that you want to
key out, and the foreground images you want to preserve. A View pop-up
menu allows you to look at the source of the clip (with no key applied),
the matte created by the filter, the final matted image, or a special
composite of the source, matte, and final image for reference. A Key
Mode pop-up menu allows you to specify whether this filter keys out
brighter, darker, similar, or dissimilar areas of the image. A Matte
pop-up menu lets you create either alpha channel information for that
clip, or a high-contrast matte image applied to the color channels of
your clip, based on the matte created by this filter.
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